A Glan–Thompson prism consists of two right-angled
calcite prisms that are cemented together by their long faces. The
optical axes of the calcite crystals are parallel and aligned perpendicular to the plane of reflection.
Birefringence splits light entering the prism into two rays, experiencing different
refractive indices; the
p-polarized
ordinary ray is
totally internally reflected from the calcite–cement interface, leaving the
s-polarized
extraordinary ray to be transmitted. The prism can therefore be used as a
polarizing beam splitter. Traditionally
Canada balsam was used as the cement in assembling these prisms, but this has largely been replaced by synthetic polymers. ==Characteristics==